Osteoporotic fractures are a major source of morbidity and mortality in older women. The incidence of hip fractures in women doubles every five to six years after age 40. Vertebral crush fractures also increase with age and may result in substantial pain and decline in function. The risk of fracture increases with decreasing bone mass. Although women over 70 are at the highest risk for osteoporotic spine and hip fractures, most research has focused on prevention of fractures in younger postmenopausal women. Many women in the older age group, however, present for diagnosis and treatment of low bone mass. A fundamental question is whether treatment at this stage is beneficial. We have shown in a dose-finding study that low dose estrogen (0.25 mg/d) reduces bone resorption in older women with fewer side effects than usual dose estrogen. This proposed study will examine the effect of low dose ERT plus calcium and Vitamin D on hip and bone density compared to calcium and vitamin D alone in a randomized placebo controlled trial. If our hypotheses are correct, then a substantial decrease in fracture risk in older women could be achieved with a safe and well-tolerated regimen. Moreover, if risk factors for cardiovascular disease are also decreased, this regimen may have even greater public health benefits.